Yes you are very right the width of the lines makes a huge diff x's how many seams in a block...
It is just like the 1/4 in seam and the scant 1/4 th in seam x's how many seams.
The better the fabric and or the heaver the fabris the scant seam works best... just think 4 thickness of fabric + the thread.
Test on diff weigth's of fab and press the seam out and measure.... you realy will be surprised.
Sometimes it is just those few extra or less threads in a seam x's how many seams.